A currently popular vision of the future development of the communication in cellular networks comprises huge numbers of small autonomous devices, which typically more or less infrequently, e.g. once per week to once per minute, transmit and receive only small amounts of data or are polled for data. It is assumed that these devices are not to be associated with humans, but are rather sensors or actuators of different kinds, which communicate with application servers, which configure the devices and receive data from them, within or outside of the cellular network. Hence, this type of communication is referred to as Machine Type Communication (MTC).
The MTC related work in 3GPP has focused on MTC devices directly connected to the cellular network via the radio interface of the cellular network. However, a more prevalent scenario is that most MTC devices connect to the cellular network via a gateway. In this scenario, the gateway acts like a user equipment (UE) towards the cellular network while maintaining a local network that is typically based on a short-range radio (SRR) technology towards the MTC devices.
Such a local network, which extends the range of the cellular network to other radio technologies, is denoted capillary network and the gateway connecting the capillary network to the cellular network is referred to as a capillary network gateway (CGW). Radio technologies that are expected to be common in capillary networks include e.g. IEEE 802.15.4 (e.g. with 6LoWPAN or ZigBee as the higher layers), Bluetooth Low Energy (BLE) or protocols in the IEEE 802.11 family (i.e. Wi-Fi).
The CGW can be under the control of the operator of the cellular network irrespective of whether the cellular network operator or some other party, such as the owner/operator of the capillary network, owns the CGW.
Low power operation is a critical requirement in the design of protocol solutions for SRR technologies.
The classic design approach for SRR includes two classes of devices. A first class are full functionalities devices (FFDs) with high computational capabilities and responsibilities, not subject to energy constraints, e.g. LAN access points (APs) in Wi-Fi, PAN coordinators in ZigBee, and master devices in BLE piconets. These devices are typically always on. A second class are reduced functionalities devices (RFDs) with low capabilities and responsibilities, typically mobile and battery powered with energy constraints, e.g., LAN stations (STAs), PAN nodes, and BLE slave devices. In order to save energy, these devices might be turned off while not operating.
Many algorithms have been proposed to reduce the duty cycle of RFDs so that the battery lifetime is maximized and have been included in the various SRR standards (IEEE 802.11, IEEE 802.15.4, BLE). This is typically achieved by delegating operations and functionalities to the FFDs, which do not implement duty-cycling operations.
In the context of capillary networks, there is a large number of CGWs between the SRR network and the cellular network. CGWs are often located close to each other, thus causing a wide overlap among the SRR communication ranges and, therefore, a high degree of redundant resources.
A first problem is the network energy consumption. CGWs are often continuously switched on to act as FFDs for the SRR network, thus consuming a lot of energy, even if not transmitting. The yearly amount of energy for operating capillary networks sums up considerably. In some cases the CGW may be battery based, so that energy saving is crucial.
A second aspect is related to the deployment. CGWs are typically installed once and work for long time without moving them. By doing so, the deployment should be redundant to accommodate for worst-case presence and traffic conditions of RFDs (“deploy and forget” approach). However, some of the CGWs are not needed or used for normal operations.
Finally, a dense CGW deployment causes interference and increased contention level in the downlink, with negative effects on the network performance. Even if the total downlink traffic is fixed, each CGW sends beacons and control messages that increase the probability of back-offs and collisions.